The present invention relates to a disk drive, and more particularly, to a disk drive having double heads for accessing data on a disk and a method for controlling head switching.
In general, a magnetic disk drive, such as a hard disk drive (HDD), can access a large amount of data at a high speed, and therefore is being widely used as an auxiliary storage unit for a computer system.
In a hard disk drive (HDD), data is stored on tracks arranged concentrically and radially along the magnetic disk. These tracks are accessed by a magnetic head or a data-transducer for reading out, writing, and erasing the data on the disk. This is performed as the head flies over the tracks of the disk. The head is shifted radially by the control of a head position servo mechanism capable of selectively positioning the head on certain tracks of the disk. To selectively position the head on a specific track, a current position of the head should first be identified.
The servo data indicative of the position of the head corresponding to the track is provided by a specific servo pattern read from the disk. The servo pattern is permanently written on the disk during manufacture. The servo pattern is used as track position data, and is sought by the head when accessing the data on the disk. For example, in order to provide the track position data, an embedded servo method has been used. With the embedded servo method, the servo data is alternately arranged between data regions on the disk. Servo data includes head position data, a track number (i.e., track address), and an index.
An operation to position the head on a specific track using the servo data is performed in the following two steps: one is a track seeking operation, and the other is a track following operation. With the track seeking operation the head is shifted from a current track to a desired track. With the track following operation, the head follows the desired track. Therefore, when the head is positioned on the desired track, the head follows the center line of the track to enable proper performance of the reading and writing operations. An actuator shifts the head radially along the surface of the disk in the track seeking operation, and enables the head to follow the desired track in the track following operation.
Typically, a plurality of disks are mounted in a stack by a spindle in a hard disk drive (HDD), and each disk surface corresponds to a single head. Each head reads and writes data from and on a corresponding disk. In this type of configuration, the actuator is typically combined with a single E-block assembly comprised of a plurality of support arms. A conventional E-block assembly is described in detail in U.S. Pat. No. 5,095,396 entitled Single E-block Assembly for Disk Drive issued to John S. Putnam on Mar. 10, 1992. Corresponding head gimbals assemblies (HGAs) are combined to the ends of the plurality of support arms extending from the E-block assembly. The head gimbals assemblies (HGAs) are formed such that their heads are combined with the ends of corresponding support arms by suspensions. Therefore, all heads are shifted together along the corresponding disk surfaces by the actuator.
As a separate matter, the performance of processors and memories is becoming drastically better, whereas hard disk drive (HDD) performance, in regards to disk access time, is advancing at a slower pace. For this reason, enhancing disk access time is becoming a more important goal to achieve. Disk access time is the total sum of the seek time, the latency time and the data transfer time. In a general data processing environment, the seek time required to detect a specific target track represents a large portion of the total disk access time. As a matter of fact, it represents substantially more than 50% of the total disk access time.
A conventional disk drive functions such that a head allocated to a disk surface seeks a target track on the disk, follows the target track, and reads or records data from or on the target track. Accordingly, since the head seeks the track by moving around the corresponding regions on the disk surface, the track seeking operation requires a considerable amount of time.